Office 365. Use exchange to send out e-mail. It costs $ 4,50/month for an account. Office 365 offers exchange mailboxes as well as POP3 and SMTP connections. The exchange mailboxes can be accessed using Exchange Web Services.

But you might be interested in one additional option. Sendgrid is a SMTP gateway provider. They offer currently a free 25000 emails / month subscription for Windows Azure users. Sending e-mails can be done using an API or just over regular SMTP.